1. Field of the Invention
This invention relates to the bonding of wires to objects by means of ultrasonic bonding wedges. The invention has particular utility, for example, in the bonding and breaking of "larger" diameter wire used in semiconductor circuits.
2. Description of Prior Art
There are two kinds of bonding generally employed in making wire connections in semiconductor microcircuitry (such circuitry primarily comprising extremely small "chips" which incorporate very large numbers of transistors and other components, and which are readily discernible only through a microscope). The first kind is thermocompression bonding, and the second is ultrasonic bonding. Thermocompression bonding, wherein each bond is made by application of heat and pressure, is conventionally limited in its application because it is usually restricted to wires made of gold. Ultrasonic bonding is more important and has a much wider application, being usable with respect to various metals including (for example) aluminum, gold, platinum, copper and nickel. The two types of bonding are thus distinctly different from each other not only in manner of operation but in field of use. The present invention relates only to the second type, namely ultrasonic.
U.S. Pat. No. 3,460,238, assigned to Motorola, Inc., describes an ultrasonic bonding apparatus wherein the bonding wedge (the tool portion which actually engages the wire) is itself employed to break the wire after completion of formation of the second ultrasonic bond. This is accomplished by re-engaging the wedge with a wire portion adjacent the already-completed second bond, and then shifting the wedge away from the bond so that frictional forces will apply tension to the wire and thus break it.
As stated in column 1 of such U.S. Pat. No. 3,460,238, "it is necessary to sever the wire after making one of the two bonds. To date, such wire severing has been performed by apparatus such as clamps which may pull the wire for breaking it at the bonding area. Other bonding machines include a scissors type of operation for cutting the wire immediately adjacent to the bond area which may leave a short undesirable tab at the bonding area."
An example of the "pulling" approach to post-bonding wire breaking is to be found in General Motors U.S. Pat. No. 3,689,983, column 5, which states: "The unbonded wire may be separated from the interconnection leaving a tailless bond by a method now to be described. While the tip remains in pressing engagement with bond 30 of the final bond of an interconnection, the unbonded wire can be pulled along channel 44 and groove 41. It has been found that the separation occurs generally within a portion of the bond underlying the groove area. Accordingly, a portion designated 46 of the bond 30 is thus removed, providing a tailless bond."
An example of both the pulling and the "scissors" approach to post-bonding wire breaking is to be found in IBM U.S. Pat. No. 3,646,307, column 7, where it is stated: "If any last bonds terminate an interconnection, feeding tip 80 is retracted with the wires clamped until sufficient force is exerted to break the wires off adjacent bond tip 134. Provision for notching wires adjacent bonding tip 134 may be made when the nature of the wires is such that they do not easily break off."
U.S. Pat. No. 3,087,239, assigned to Western Electric, teaches the use of a scissors device 13 in a thermocompression bonding apparatus, such patent stating (column 2) that "the invention is not limited to the forming of thermo-compression bonds only. For example, mechanical vibrations at ultrasonic frequencies may be utilized as a bonding aid in place of the heat employed in the instant embodiment." As an alternative to the scissors device 13, the patent (column 3) states: "Other means for shearing lead 12 may be utilized. For example, referring to FIG. 1C, pressure greater than that required to provide a bond may be applied by tip 17 thereby to bond and break through the lead in one step. Shearing device 13 is preferred, however, because it combines the shearing and orienting steps."
There are various major disadvantages to the above-mentioned (and other) approaches used by prior-art workers, particularly when larger diameter wires are being bonded (as is the case, for example, in semiconductor power circuitry). The word larger is placed in quotation marks because the wire may still be extremely small, for example about two to four thousandths of an inch in diameter. It is to be understood that the present invention is also applicable to wires having diameters much larger than four thousandths of an inch.
Relative to post-bonding pulling methods of wire breaking, these tend to damage the wire in one or both of two locations--at the bond and/or at the wire clamp. If the wire clamp, for example, clamps sufficiently tightly to pull adequately on the larger diameter wires, without slipping, there may result marks or nicks, etc., at the point of clamping. Such marks or nicks are believed by the present inventor to be responsible for service problems because they localize stress and--during temperature cycling-- may propagate cracks leading to failure of the wire. Similarly, if the bonding wedge is clamped down on the previously-formed bond with sufficient force to prevent it from pulling loose in response to tensioning of the wire, there may result deleterious marks and nicks at the upper regions of the bonded wire end.
Relative to the shearing or scissors approach to wire breaking, there are different problems depending upon what type of shearing is employed. If the shearing is removed from the bond, there is a "tail" adjacent the bond which is distinctly undesirable. Tailless wire bonding has therefore been extensively performed by means of a chopper blade located just ahead of the front edge of the bonding wedge. After competion of the second bond, the wedge is lifted and moved back the length of the bond. At this point, the chopper blade moves down and severs the wire at the back of the bond. Such chopping, however, creates shear forces which are transmitted to the bond and may damage it.
It is emphasized that no system for breaking the wire should be such as to injure the underlying element (usually a pad or post) to which the wire is being bonded. Therefore, and to minimize dulling of the blade, chopper blades are usually-- insofar as it can be accomplished--so adjusted that they never cut completely through the wire. In practice, however, such adjustment is difficult because of manufacturing tolerances which exist relative to the placing of the bonding plane.
Insofar as applicant is aware, all commercially used wire-breaking steps follow the bonding step. They therefore slow the operation, requiring a portion of the machine cycle time. The above-cited U.S. Pat. No. 3,087,239, which relates primarily to thermocompression welding, would if the stated "other means" (column 3, first full paragraph) were employed tend to cause squashing of the bond and injury to the underlying pad or post.
There has long existed a clear need for fast severing of ultrasonically bonded wires in a tailless manner, with no substantial possibility of injury to the wire or the work. This need is evidenced by a study of the above-mentioned (and other) prior-art patents.